Page 14 - MFWDec 2023

Scale build of

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7Z
7Z
OBRA AH-1 N1
OBRA AH-1 N1
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C C
COBRA AH-1 N107Z
Chris Ball
I prefer non-military helicopters to copy, but this time I had the
opportunity to get the best of both worlds, a 1969 Bell AH-1 attack
Cobra that the U.S. Forest Service acquired 25 of from the military.
They were stripped of all the battle stuff and fitted with FLIR’s and other
electronics for heat seeking of forest fires, with this information related
to the firefighting helicopters and planes. Of those, two almost identical
Cobras, N109Z and N107Z, were retired after their last flight Saturday
October 16, 2021. It was those that caught my eye, and it was N107Z I
finally chose to copy as a 2 metre long scale build.

N107Z has a striking livery, with Fire Truck red, a vibrant white, and a
dash of black by way of ‘V’s on the fuselage body, a strip of black on the
tail boom and matt black on the nose.
Above:
Like my Bell 204, I started with electric-powered Align 800 mechanics Original Vario nose.
with chopped down main frames so that the tail boom aligned correctly Left:
with the 45 degree angle tail gearbox. The downside of that meant that Shaft and guide
the mechanics had to sit lower in the fuselage than desired, and the extensions.
necessity for a long main shaft to be sourced and modified so that the Below:
rotor head sat at the correct height above the doghouse. This Black anodized
rotor head.
introduced another problem, the extra support needed by way of an
extension to place the top shaft bearing some 100mm higher than in Bottom:
Cross bracing and
the standard Align 800. I drew up the design and had my ‘tame guru’ mechanics
machine this from 31mm diameter/ 5mm wall thickness aluminium supports.
tube, reinforce this internally at the base with a collar for extra bolt
strength, and mill for the top bearing to be moved from its original
position to the top of the new extension. This, along with the solid
12mm diameter main shaft, should give sufficient support to the three
main bearings.

Then I had to design and make a longer anti-rotation
guide, use longer servo to swash and swash to pitch arm
links and add a follower to retain the swash in the desired
position. All done now.

The bare and unpainted fuselage is 2 metres long from tip
to tip (1.7 scale) from VARIO in Germany. They make
excellent hulls and this is no exception, but the nose was
designed to take a scale gatling gun, and the nose belly is
physically different to N107Z. I had to shave quite a lot off
both front sides of the nose and add extra parts behind that
to get the correct ‘look’. Then there was the normal cutting
of vents, create a hinged door to fit the switches, make and
fit inspection plates…..the list goes on.

With the mechanic sitting low, the fuselage around 40cm I am using the standard Align 2-blade rotor head. It is not
high (without skids) and the rotor head above that, it was perfect to scale, but close enough, plus there is no
important to add internal ply and aluminium structural commercially made head that would do a better job. This
support. Only then would the two work together as one has been sandblasted and black anodized to match the
unit. This was fitted and glued in place, and an extra colour of the real helicopter.
aluminium cross brace was made to bolt between the port
and starboard main structures with a small, bolted plate The standard Align gearing has been retained, but to
that fits snuggly between the main frames to prevent achieve the desired head speed of around 1270 RPM, the
lateral movement. It works! motor will be a hand wound EGODRIFT high torque
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